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譯文題目： Friction And Lubrication
摩擦與潤滑
學(xué)生姓名： 學(xué) 號：
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20xx年 2月 27日
Friction And ?Lubrication
In?many?of?the?problem?thus?far?,?the?student?has??been?asked?to?disregard?or?neglect?friction?.?Actually?,?friction?is?present?to?some?degree?whenever?two?parts?are?in?contact?and?move?on?each?other.?The?term?friction?refers?to?the?resistance?of?two?or?more?parts?to?movement.?
Friction?is?harmful?or?valuable?depending?upon?where?it?occurs.?friction??is??necessary?for?fastening?devices?such?as?screws?and?rivets?which?depend?upon?friction?to?hold?the?fastener?and?the?parts?together.?
The?friction?of?moving?parts?in?a?machine?is?harmful?because?it?reduces?the?mechanical?advantage?of?the?device.?The?heat?produced?by?friction?is??lost?energy?because?no?work?takes?place.?Also?,?greater?power?is?required?to?overcome?the??increased?friction.?Heat?is?destructive?in?that?it?causes?expansion.?Expansion?may?cause?a?bearing?or?sliding?surface?to?fit?tighter.?
There?are?three?types?of?friction?which?must?be?overcome?in?moving?parts:?(1)starting,?(2)sliding,?and(3)rolling.?Starting?friction?is?the?friction?between?two?solids?that?tend?to?resist?movement.?When?two?parts?are?at?a?state?of?rest,?the?surface?irregularities?of?both?parts?tend?to?interlock?and?form?a?wedgingaction.?To?produce?motion?in?these?parts,?the?wedgeshaped?peaks?and?valleys?of?the?stationary?surfaces?must?be?made?to?slide?out?and?over?each?other.?The?rougher?the?two?surfaces,?the?greater?is?starting?friction?resulting?from?their?movement?.
Since?there?is?usually?no?fixed?pattern?between?the?peaks?and?valleys?of?two?mating?parts,?the?irregularities?do?not?interlock?once?the?parts?are?in?motion.?The?friction?of?the?two?surfaces?is?known?as?sliding?friction.?As?shown?in?figure?,starting?friction?is?always?greater?than?sliding?friction?.
Rolling?friction?occurs?when?roller?devces?are?subjected?to?tremendous?stress?which?cause?the?parts?to?change?shape?or?deform.?Under?these?conditions,?the?material?in?front?of?a?roller?tends?to?pile?up?and?forces?the?object?to?roll?slightly?uphill.?This?changing?of?shape?,?known?as?deformation,?causes?a?movement?of?molecules.?As?a?result?,heat?is?produced?from?the?added?energy?required?to?keep?the?parts?turning?and?overcome?friction.?
The?friction?caused?by?the?wedging?action?of?surface?irregularities?can?be?overcome?partly?by?the?precision?machining?of?the?surfaces.?However,?even?these?smooth?surfaces?may?require?the?use?of?a?substance?between?them?to?reduce?the?friction?still?more.?This?substance?is?usually?a?lubricant?which?provides?a?fine,?thin?oil?film.?The?film?keeps?the?surfaces?apart?and?prevents?the?cohesive?forces?of?the?surfaces?from?coming?in?close?contact?and?producing?heat?.?
Another?way?to?reduce?friction?is?to?use?different?materials?for?the?bearing?surfaces?and?rotating?parts.?This?explains?why?bronze?bearings,?soft?alloys,?and?copper?and?tin?iolite?bearings?are?used?with?both?soft?and?hardened?steel?shaft.?The?iolite?bearing?is?porous.?Thus,?when?the?bearing?is?dipped?in?oil,?capillary?action?carries?the?oil?through?the?spaces?of?the?bearing.?This?type?ofbearing?carries?its?own?lubricant?to?the?points?where?the?pressures?are?the?greatest.
Moving?parts?are?lubricated?to?reduce?friction,?wear,?and?heat.?The?most?commonly?used?lubricants?are?oils,?greases,?and?graphite?compounds.?Each?lubricant?serves?a?different?purpose.?The?conditions?under?which?two?moving?surfaces?are?to?work?determine?the?type?of?lubricant?to?be?used?and?the?system?selected?for?distributing?the?lubricant.?
On?slow?moving?parts?with?a?minimum?of?pressure,?an?oil?groove?is?usually?sufficient?to?distribute?the?required?quantity?of?lubricant?to?the?surfaces?moving?on?each?other?.?
A?second?common?method?of?lubrication?is?the?splash?system?in?which?parts?moving?in?a?reservoir?of?lubricant?pick?up?sufficient?oil?which?is?then?distributed?to?all?moving?parts?during?each?cycle.?This?system?is?used?in?the?crankcase?of?lawn-mower?engines?to?lubricate?the?crankshaft,?connecting?rod?,and?parts?of?the?piston.?
A?lubrication?system?commonly?used?in?industrial?plants?is?the?pressure?system.?In?this?system,?a?pump?on?a?machine?carries?the?lubricant?to?all?of?the?bearing?surfaces?at?a?constant?rate?and?quantity.?
There?are?numerous?other?systems?of?lubrication?and?a?considerable?number?of?lubricants?available?for?any?given?set?of?operating?conditions.?Modern?industry?pays?greater?attention?to?the?use?of?the?proper?lubricants?than?at?previous?time?because?of?the?increased?speeds,?pressures,?and?operating?demands?placed?on?equipment?and?devices.?
Although?one?of?the?main?purposes?of?lubrication?is?reduce?friction,?any?substance-liquid?,?solid?,?or?gaseous-capable?of?controlling?friction?and?wear?between?sliding?surfaces?can?be?classed?as?a?lubricant.
Varieties?of?lubrication
Unlubricated?sliding.?Metals?that?have?been?carefully?treated?to?remove?all?foreign?materials?seize?and?weld?to?one?another?when?slid?together.?In?the?absence?of?such?a?high?degree?of?cleanliness,?adsorbed?gases,?water?vapor?,oxides,?and?contaminants?reduce?frictio9n?and?the?tendency?to?seize?but?usually?result?in?severe?wear;?this?is?called?“unlubricated?”or?dry?sliding.?
Fluid-film?lubrication.?Interposing?a?fluid?film?that?completely?separates?the?sliding?surfaces?results?in?fluid-film?lubrication.?The?fluid?may?be?introduced?intentionally?as?the?oil?in?the?main?bearing?of?an?automobile,?or?unintentionally,?as?in?the?case?of?water?between?a?smooth?tuber?tire?and?a?wet?pavement.?Although?the?fluid?is?usually?a?liquid?such?as?oil,?water,?and?a?wide?range?of?other?materials,?it?may?also?be?a?gas.?The?gas?most?commonly?employed?is?air.?
Boundary?lubrication.?A?condition?that?lies?between?unlubricated?sliding?and?fluid-film?lubrication?is?referred?to?as?boundary?lubrication,?also?defined?as?that?condition?of?lubrication?in?which?the?friction?between?surfaces?is?determined?by?the?properties?of?the?surfaces?and?properties?of?the?lubricant?other?than?viscosity.?Boundary?lubrication?encompasses?a?significant?portion?of?lubrication?phenomena?and?commonly?occurs?during?the?starting?and?stopping?off?machines.?
Solid?lubrication.?Solid?such?as?graphite?and?molybdenum?disulfide?are?widely?used?when?normal?lubricants?do?not?possess?sufficient?resistance?to?load?or?temperature?extremes.?But?lubricants?need?not?take?only?such?familiar?forms?as?fats,?powders,?and?gases;?even?some?metals?commonly?serve?as?sliding?surfaces?in?some?sophisticated?machines.
Function?of?lubricants
Although?a?lubricant?primarily?controls?friction?and?ordinarily?does?perform?numerous?other?functions?.?
Friction?control.?The?amount?and?character?of?the?lubricant?made?available?to?sliding?surfaces?have?a?profound?effect?upon?the?friction?that?is?encountered.?For?example,?disregarding?such?related?factors?as?heat?and?wear?but?considering?friction?alone?between?the?same?surfaces?with?on?lubricant.?Under?fluid-film?conditions,?friction?is?encountered.?In?a?great?range?of?viscosities?and?thus?can?satisfy?a?broad?spectrum?of?functional?requirements.?Under?boundary?lubrication?conditions?,?the?effect?of?viscosity?on?friction?becomes?less?significant?than?the?chemical?nature?of?the?lubricant.? Wear?control.?wear?occurs?on?lubricated?surfaces?by?abrasion,?corrosion?,and?solid-to-solid?contact?wear?by?providing?a?film?that?increases?the?distance?between?the?sliding?surfaces?,thereby?lessening?the?damage?by?abrasive?contaminants?and?surface?asperities.?
Wear?control.?wear?occurs?on?lubricated?surfaces?by?abrasion,?corrosion?,and?solid-to-solid?contact?wear?by?providing?a?film?that?increases?the?distance?between?the?sliding?surfaces?,thereby?lessening?the?damage?by?abrasive?contaminants?and?surface?asperities.
Temperature?control.?Lubricants?assist?in?controlling?corrosion?of?the?surfaces?.?When?machinery?is?idle,?the?lubricant?acts?as?a?preservative.?When?machinery?is?in?use,?the?lubricant?controls?corrosion?by?coating?lubricated?parts?with?a?protective?film?that?may?contain?additives?to?neutralize?corrosive?materials.?The?ability?of?a?lubricant?to?control?corrosion?is?directly?relatly?to?the?thickness?of?the?lubricant?film?remaining?on?the?metal?surfaces?and?the?chermical?composition?of?the?lubricant.?
Other?functions
Lubrication?are?frequently?used?for?purposes?other?than?the?reduction?of?friction.?Some?of?these?applications?are?described?below.?
Power?transmission.?Lubricants?are?widely?employed?as?hydraulic?fluids?in?fluid?transmission?devices.?
Insulation.?In?specialized?applications?such?as?transformers?and?switchgear?,?lubricants?with?high?dielectric?constants?acts?as?electrical?insulators.?For?maximum?insulating?properties,?a?lubricant?must?be?kept?free?of?contaminants?and?water.? ?
Sealing.?Lubricating?grease?frequently?performs?the?special?function?of?forming?a?seal?to?retain?lubricants?or?to??exclude?contaminants.?
Hydrodynamic?lubrication?means?that?the?load-carrying?surfaces?of?the?bearing?are?separated?by?a?relatively?thick?film?of?lubricant,?so?as?to?prevent?metal?contact,?and?that?the?stability?thus?obtained?can?be?explained?by?the?laws?of?the?lubricant?under?pressure??.
Hydrostatic?lubrication?is?obtained?by?introducing?the?lubricant?,which??is?sometime?air?or?water?,into?the?load-bearing?area?at?a?pressure?high?enough?to?separate?the?surface?with?a?relatively?thick?film?of?lubricant.?So?,unlike?hydrodynanmic?lubrication,?motion?of?one?surface?relative?to?another?is?not?required?.
The?understanding?of?boundary?lubrication?is?normally?attributed?to?hardy?and?doubleday?,?who?found?the?extrememly?thin?films?adhering?to?surfaces?were?often?sufficient?to?assistrelative?sliding.?They?concluded?that?under?such?circumstances?the?chemical?composition?of?fluid?is?important,?and?they?introduced?the?term?“boundary?lubrication”.?Boundary?lubrication?is?at?the?opposite?end?of?the?spectrum?from?hydrodynamic?lubrication.
Elasohydrodynamic?lubrication?is?the?phenomenon?that?occurs?when?a?lubricant?is?introduced?between?surfaces?which?are?in?rolling?contact,?such?as?mating?gears?or?rolling?bearings.?The?mathematical?explanation?requires?the?hertzian?theory?of?contact?stress?and?fluid?mechanics.
The?object?of?lubrication?is?to?reduce?friction?,wear?,?and?heating?of?machine?pars?which?move?relative?to?each?other.?A?lubricant?is?any?substance?which,?when?inserted?between?the?moving?surfaces,?accomplishes?these?purposes.?Most?lubricants?are?liquids(such?as?mineral?oil,?silicone?fluids,?and?water),but?they?may?be?solid?for?use?in?dry?bearings,?greases?for?use?in?rolling?element?bearing,?or?gases(such?as?air)?for?use?in?gas?bearings.?The?physical?and?chemical?interaction?between?the?lubricant?and?lubricating?surfaces?must?be?understood?in?order?to?provide?the?machine?elements?with?satisfactory?life.?
摩擦與潤滑
現(xiàn)在看來，有很多這種情況，許多學(xué)生在被問到關(guān)于摩擦的問題時，往往都沒引起足夠的重視，甚至是忽視它。實(shí)際上，摩擦從某種程度上說，存在于任何兩個相接觸并有相對運(yùn)動趨勢的部件之間。而摩擦這個詞，本身就意味著，兩個或兩個以上部件的阻止相對運(yùn)動趨勢。
摩擦是有害的還是有益的，關(guān)鍵在于它作用在什么地方。對于像螺絲和鉚釘這些依靠摩擦固定緊固體和機(jī)械部件的緊固結(jié)構(gòu)來說，摩擦是必要的。?
在一個機(jī)器中，運(yùn)動部件的摩擦是有害的，因?yàn)樗档土藱C(jī)械對能量的充分利用。由它引起的熱能是一種浪費(fèi)的能量，因?yàn)樗蛔鋈魏喂?。而且，它還需要更大的動力來克服這種不斷增大的摩擦。熱能是有破壞性的，因?yàn)樗a(chǎn)生了膨脹。而膨脹可以使得軸承或滑動表面之間的配合更緊密。
有三種摩擦是在運(yùn)動中必須被克服的：（?1）啟動摩擦（?2）滑動摩擦（?3）轉(zhuǎn)動摩擦。?啟動摩擦是兩個固體之間產(chǎn)生的傾向于抵抗其相對運(yùn)動趨勢的摩擦。當(dāng)兩個固體處于靜止?fàn)顟B(tài)時，這兩個零件表面的不規(guī)則處傾向于相互嵌入，形成楔入作用。為了使這些部件“動”起來，這些固定表面的楔形峰和谷必須制作光滑，而且能相互抵消。這兩個表面之間越粗糙，由運(yùn)動造成的啟動摩擦（最大靜摩擦力）就會越大。?
因?yàn)?，通常來說，在兩個相互配合的部件之間，其表面不規(guī)則沒有固定的圖形，當(dāng)這些部件在運(yùn)動時，不規(guī)則處不能相互配合。兩個表面之間產(chǎn)生的摩擦就是我們所知的滑動摩擦。由計算可知，啟動摩擦通常都稍大于滑動摩擦。?
當(dāng)輥裝置承受引起機(jī)構(gòu)形變或變形的巨大壓力時，轉(zhuǎn)動摩擦就產(chǎn)生了。在這些情況下，轉(zhuǎn)動件的材料趨向于堆積并且強(qiáng)迫運(yùn)動部件緩慢運(yùn)動，這種引起分子運(yùn)動的形狀的改變就是通常所說的形變。當(dāng)然，最終的結(jié)果是，這種必須的保證運(yùn)動部件運(yùn)動和克服摩擦力的額外的能量產(chǎn)生了熱能。?
由于運(yùn)動部件表面不規(guī)則的楔入作用而引起的摩擦可以通過兩表面之間的潤滑被部分的克服。但是，即使是非常光滑的兩個表面之間也可能會使用一種材料去減少摩擦。這種材料通常是一種潤滑劑，它可以提供一個比較好的、比較薄的油膜。這個油膜使兩個表面分離，并且阻止運(yùn)動部件的兩個表面之間的凝聚力使它們更近的接觸并產(chǎn)生熱量。?
減小摩擦的另一種方式是用不同的材料制造軸承和轉(zhuǎn)動零件。這就解釋了為什么黃銅軸承、鋁合金和含油軸承合金都使用軟的或硬的金屬組成表面。含油軸承合金有較多孔洞。因此，當(dāng)軸承在油中浸泡過以后，因?yàn)槊?xì)管的作用，將油帶到軸承的各個表面。這種類型的軸承把它的潤滑劑帶到應(yīng)力最大的部位。?
對運(yùn)動部件潤滑以減小摩擦，應(yīng)力和熱量。最常用的潤滑劑是油、脂、還有合成劑。每一種潤滑劑都有其各自不同的功能和用途。兩個運(yùn)動部件之間的運(yùn)動情況決定了潤滑劑的類型的選擇，并且系統(tǒng)也被選擇來適應(yīng)這種潤滑劑。?
在承受最小壓力低速度運(yùn)動的部件中，一個油溝足以將所需要的數(shù)量的潤滑劑送到相互運(yùn)動的表面。?
第二種通用的潤滑方法是飛濺潤滑系統(tǒng)，在每個周期內(nèi)這個系統(tǒng)內(nèi)一些零件經(jīng)過潤滑劑存儲的位置，帶起足夠的潤滑油，然后將其散布到所有的運(yùn)動零件上。這種系統(tǒng)用于草坪修剪機(jī)中發(fā)動機(jī)的曲軸箱，對曲軸、連桿和活塞等零件進(jìn)行潤滑。?
在工業(yè)裝置中，常用的有一種潤滑系統(tǒng)是壓力系統(tǒng)。這種系統(tǒng)中，一個機(jī)器上的一個泵，可以將潤滑劑以一種連續(xù)固定的速度和數(shù)量帶到所有的軸承表面。?
關(guān)于潤滑，還有許多其他的系統(tǒng)。有可觀數(shù)量的潤滑劑可被運(yùn)用在任意一種給定的操作狀況中。由于設(shè)備或裝置的速度、壓力和工作要求的提高，現(xiàn)代工業(yè)比以前任何時候都更注重選用適當(dāng)?shù)臐櫥瑒?
盡管潤滑的主要目的之一是為了減小摩擦力，任何可以控制兩個滑動表面之間摩擦和磨損的物質(zhì)，不管是液體還是?固體或氣體，都可以歸類于潤滑劑。?
潤滑的種類
無潤滑滑動。經(jīng)過精心處理的、去除了所有外來物質(zhì)的金屬在相互滑動時會粘附或熔接到一起。當(dāng)達(dá)不到這么高的純凈度時，吸附在表面的氣體、水蒸氣、氧化物和污染物就會降低摩擦力并減小粘附的趨勢，但通常會產(chǎn)生嚴(yán)重的磨損，這種現(xiàn)象被稱為“無潤滑”摩擦或者叫做干摩擦。?
流體膜潤滑。在滑動面之間引入一層流體膜，把滑動表面完全隔離開，就產(chǎn)生了流體膜潤滑。這種流體可能是有意引入的。例如汽車主軸承中的潤滑油；也可能是無意中引入的，例如在光滑的橡膠輪胎和潮濕的路面之間的水。盡管流體通常是油、水和其他很多種類的液體，它可以是氣體。最常用的氣體是空氣。?
邊界潤滑。處于無潤滑滑動和流體膜潤滑之間的潤滑被稱為邊界潤滑。它可以被定為這樣一種潤滑狀態(tài)，在這種狀態(tài)中，表面之間的摩擦力取決于表面的性質(zhì)和潤滑劑中的其他性質(zhì)。邊界潤滑包括大部分潤滑現(xiàn)象，通常在機(jī)器的啟動和停止時出現(xiàn)。?
固體潤滑。當(dāng)普通潤滑劑沒有足夠的承受能力或者不能在溫度極限下工作時，石墨和二硫化鉬這一類固體潤滑劑得廣泛運(yùn)用。但是潤滑劑并不是只有油脂，粉末和氣體這幾種熟悉的形式。在一些靜密的儀器中，甚至一些金屬一般也能也能作為滑動表面。
潤滑劑的作用
盡管潤滑劑根本上是為了控制摩擦，但一般也起著許多其他的作用。
摩擦控制。滑動表面可用潤滑劑的數(shù)量和特征對其產(chǎn)生的摩擦有著深遠(yuǎn)的影響。舉個例子，無視像熱量和壓力這樣的相關(guān)因素，只考慮在潤滑劑作用下相同表面的摩擦。在液膜狀態(tài)下，摩擦是存在的。在一個大的粘度范圍內(nèi)，從而能夠滿足廣泛的功能要求。在邊界潤滑條件下，粘度對摩擦的影響變得顯著低于化學(xué)性質(zhì)的潤滑劑。
磨損控制。磨損通過磨蝕、腐蝕和固體與固體的接觸作用于潤滑表面。壓力通過提供一個膜來增加滑動表面之間的距離，從而減少磨料污染物和表面粗糙的破壞。
溫度控制。潤滑劑協(xié)助控制腐蝕的表面。當(dāng)機(jī)器閑置時，潤滑劑可作為防腐劑使用。當(dāng)機(jī)械在使用中時，潤滑劑通過具有保護(hù)膜的、含有中和腐蝕性材料的添加劑的涂層潤滑部分來控制腐蝕。潤滑劑控制腐蝕的能力與保留在金屬表面的潤滑膜厚度和潤滑劑的化學(xué)成分直接相關(guān)。
其它作用
潤滑除了用于減少摩擦還經(jīng)常用于其它目的。其中的一些應(yīng)用如下所述。
能量傳輸。潤滑油被廣泛用作液壓流體在流體傳輸設(shè)備中使用。
絕緣。在如變壓器、開關(guān)設(shè)備這些專門的應(yīng)用中，具有高階電常數(shù)的潤滑油廠用作電器絕緣體。為了保持最大的絕緣特性，潤滑劑必須遠(yuǎn)離污染物和水。
密封。潤滑脂經(jīng)常被用作一些特殊的功能，形成一個密封空間從而保存潤滑劑或者排除污染物。
液體動力潤滑意味著軸承的承載面有著相對厚的潤滑膜，可以防止金屬接觸，獲得在潤滑壓力下可以接受的良好穩(wěn)定性。
靜壓潤滑是通過引入潤滑劑，有時是空氣或水，到壓力足夠高能夠用較厚的潤滑膜去分離表面的承載區(qū)域。所以，它不需要像液體動力潤滑那樣，從一個表面運(yùn)動到另一個表面。
對于邊界潤滑的理解主要?dú)w功于哈布和道布爾迪，他們發(fā)現(xiàn)依附于表面的極度薄的潤滑膜通常已經(jīng)足夠用于相對滑動。他們得出的結(jié)論是，在這種情況下流體的化學(xué)成分是很重要的，他們介紹了術(shù)語“邊界潤滑”。
非穩(wěn)態(tài)彈流潤滑現(xiàn)象發(fā)生在當(dāng)潤滑劑在兩個表面之間滾動接觸時，如齒輪配合和滾動軸承。數(shù)學(xué)解釋要求赫茲接觸應(yīng)力和流體力學(xué)的理論。
潤滑的目的是為了減少機(jī)械相對運(yùn)動部分產(chǎn)生的摩擦，磨損和熱量。潤滑劑是在移動表面之間實(shí)現(xiàn)這些目的的任何物質(zhì)。大多數(shù)潤滑劑是液體（如礦物油、有機(jī)硅液體和水），但它們也可能是用于干燥軸承的固體、用于滾動軸承的油脂、或者是用于氣體軸承的氣體（如空氣）。潤滑劑和潤滑表面之間的物理和化學(xué)作用必須理解為為了使機(jī)器有令人滿意的工作狀態(tài)。